1. Field
Aspects of the present disclosure relate generally to wireless communication systems, and more particularly, to decoding wireless communications.
2. Background
Wireless communication networks are widely deployed to provide various communication services such as telephony, video, data, messaging, broadcasts, and so on. Such networks, which are usually multiple access networks, support communications for multiple users by sharing the available network resources. One example of such a network is the UMTS Terrestrial Radio Access Network (UTRAN). The UTRAN is the radio access network (RAN) defined as a part of the Universal Mobile Telecommunications System (UMTS), a third generation (3G) mobile phone technology supported by the 3rd Generation Partnership Project (3GPP). The UMTS, which is the successor to Global System for Mobile Communications (GSM) technologies, currently supports various air interface standards, such as Wideband-Code Division Multiple Access (W-CDMA), Time Division-Code Division Multiple Access (TD-CDMA), and Time Division-Synchronous Code Division Multiple Access (TD-SCDMA). The UMTS also supports enhanced 3G data communications protocols, such as High Speed Packet Access (HSPA) and similar derivatives, which provide higher data transfer speeds and capacity to associated UMTS networks.
User equipment (UE) in some wireless systems can employ adaptive chip-rate equalizers (EQ) to improve high speed data throughput performance. T-spaced equalizers (also referred to as cx1 EQ), for example, operate on “chip-rate-one” receive samples. Using a circulant approximation of covariance matrix, the cx1 EQ weights (also known as the tap coefficients or tap weights) are often computed in the frequency domain by dividing the Discrete Fourier Transform (DFT) of the channel impulse response (CIR) by the DFT of the covariance of the received samples. Although the estimated CIR and covariance values are corrupted by measurement noise, this division operation does not pose significant ill-conditioning issues (such as division by zero) due to the inherent aliasing components present in the cx1 samples.
The cx1 EQ performance is sensitive to the phase of the cx1 sampling clock, and selection of the optimum sampling phase in a fading channel scenario can often be impractical. Hence, one version of a cx1 EQ, referred to as a branch select cx1 EQ, can select one of two sampling phases, spaced at half of a chip distance (on-time or late), based on the corresponding CIR energies to obtain a sub-optimum cx1 EQ output. A UE can use another EQ, such as a (T/2)-fractionally spaced equalizer (also referred to as a cx2 EQ), which uses a front-end root-raised-cosine (RRC) filter to obtain a chip estimate from on-time and late branches. A cx1 EQ may nevertheless exhibit inferior performance to a cx2 EQ because the cx2 rate satisfies the Nyquist criterion for reconstructing band-limited signals. Computation of the cx2 tap coefficients based on the optimum minimum mean square error (MMSE) criterion, however, can lead to ill-conditioning issues due to the combined effects of the measurement noise mentioned above and the band limiting caused by the front-end RRC filter.